Plasma dry etch machines on reactors are vacuum chambers in which an electrical plasma is created in order to etch semiconductor wafers. The etching is usually performed through a photoresist mask. Quite often it is desired to etch oxide layers preferentially over adjacent polysilicon such as the wafer substrate.
In the dry etch process, a wafer is placed over a metallic anode plate and plasma is discharged to flow from a cathode to the anode plate, where the wafer functions as the anode. This process causes the wafer to increase in temperature, usually to about 100 to 150 degrees C. The anode plate is cooled in order to cool the wafer; using cooling water, it is typically at about 20 degrees C. The cooled anode plate thereby cools the wafer. If the wafer is permitted to elevate in temperature, the reaction is adversely affected. More importantly, photoresist tends to be more stable at lower temperatures, and at higher temperatures tends to erode or change form or composition.
In the etching process, a fluoropolymer is applied to the wafer in order to cause the preferential etching of silicon dioxide over silicon. The fluoropolymer tends to deposit on the anode plate, particularly at cool portions of the anode plate.
This deposition necessitates a removal of the anode plate for cleaning after a given number of cycles, typically 100 cycles. The removal for cleaning results in equipment downtime for the plasma dry etch machine because of the difficulty in removing and replacing the anode plate. If the anode plate is maintained at a particularly cool temperature, the fluoropolymer tends to deposit more rapidly, therefore, while efficient operation of the anode plate can be achieved by operating the anode plate at higher temperatures, the wafer must be maintained at low temperatures.
Polymer buildup tends to occur around the outer perimeter of the anode outside of the area where the wafer is resting, but this buildup interferes with the etching process. The necessity to maintain the wafer at low temperatures also implies that power applied in the plasma etch process be limited. Higher power would result in a higher etch rate and better throughput, but it is necessary to restrict power in order that the surface temperature of the wafer does not exceed the temperature for deterioration of the photoresist.
For this reason, it is desired that an anode design be able to accommodate a more accurate control of temperature and a greater cooling rate. It is also desired that the anode be able to be operated for longer periods of time between cleaning processes. It is therefore desirable that the anode be able to transfer heat from a silicon wafer rapidly, while being less susceptible to polymer buildup.